Methods and Compositions for Altering the Viscosity of Treatment Fluids Used in Subterranean Operations

ABSTRACT

Methods of decreasing the viscosity of a treatment fluid through contact with a cyclodextrin modifier. Such methods include providing a cyclodextrin modifier; providing a treatment fluid that comprises a base fluid and a viscosifying agent wherein the viscosifying agent is selected from the group consisting of a hydrophobically modified polymer, a viscoelastic surfactant, a phosphonate surfactant, or a combination thereof; and, introducing the cyclodextrin modifier and the treatment fluid into a well bore penetrating a subterranean formation wherein the viscosity of the treatment fluid is decreased due to the combination of the hydrophobically modified polymer and the cyclodextrin modifier.

BACKGROUND

The present invention relates to methods and compositions for use insubterranean operations. More particularly, in certain embodiments, thepresent invention relates to methods of altering the viscosity of asubterranean treatment fluid with a cyclodextrin modifier.

Treatment fluids may be used in a variety of subterranean operations,including, but not limited to, drilling operations, stimulation,sand-control, fluid-diversion treatments, and cementing operations. Asused herein, the term “treatment,” or “treating,” refers to anysubterranean operation that uses a fluid in conjunction with a desiredfunction and/or for a desired purpose. The term “treatment,” or“treating,” does not imply any particular action by the fluid or anyparticular component thereof.

Polymeric gelling agents, such as guar gums, cellulose derivatives,biopolymers, polysaccharides, synthetic polymers, and the like, havepreviously been added to treatment fluids to obtain a desired viscosity.Viscoelastic surfactants have also been added to treatment fluids toincrease the viscosity thereof. For example, gels can be formed by theassociation of hydrophobic portions of surfactants to form micelles orlarger associative structures. The micelles or other associativestructures increase the viscosity of the base fluid and may provideviscoelastic characteristics to the fluid in cases where the surfactantstructure is suitably chosen. Similarly, hydrophobically modified (“HM”)polymers have been utilized to increase the viscosity of aqueoustreatment fluids. As used herein, the term water-soluble relativepermeability modifier. As used herein, the term “HM polymer” refers to apolymer with hydrophobic groups incorporated into a hydrophilic polymerstructure while retaining water solubility. As used herein, a polymer isconsidered water soluble with at least 0.01 weight percent soluble indistilled water and, preferably, at least 5-10 weight percent soluble indistilled water. For instance, intermolecular associative micellar bondsmay be formed between hydrophobic groups on a different polymer chain,which result in a three-dimensional associated network, akin to across-linked network structure, that thereby increases the viscosity ofthe fluids. Surfactants may be used to promote the formation of thesemicellar bonds among HM polymer chains. As used herein, the terms“micellar associations” and “micellar bonds” refer to those associativeinteractions between hydrophobic groups on HM polymers molecules.

Maintaining sufficient viscosity in these treatment fluids may beimportant for a number of reasons. For example, maintaining sufficientviscosity is important in fracturing and sand-control treatments forparticulate transport and/or to create or enhance fracture width. Also,maintaining sufficient viscosity may be important to control and/orreduce fluid loss into the formation. Moreover, a treatment fluid of asufficient viscosity may be used to divert the flow of fluids presentwithin a subterranean formation (e.g., formation fluids, other treatmentfluids) to other portions of the formation, for example, by “plugging”an open space within the formation. At the same time, while maintainingsufficient viscosity of the treatment fluid often is desirable, it alsomay be desirable to maintain the viscosity of the treatment fluid insuch a way that the viscosity may be reduced at a particular time, interalia, for subsequent recovery of the fluid from the formation.

SUMMARY

The present invention relates to methods and compositions for use insubterranean operations. More particularly, in certain embodiments, thepresent invention relates to methods of altering the viscosity and/orfoaming characteristics of a subterranean treatment fluid with acyclodextrin modifier. As used herein, the term “cyclodextrin modifier”generally refers to cyclodextrin, cyclodextrin derivatives, cyclodextrindimers, cyclodextrin trimers, polymerized cyclodextrin, and combinationsthereof that are capable of modifying the viscosity of a treatmentfluid.

In one embodiment, the present invention provides a method comprisingintroducing a cyclodextrin modifier into a well bore penetrating asubterranean formation.

In another embodiment, the present invention provides a method ofreducing viscosity of a treatment fluid comprising contacting atreatment fluid comprising a base fluid and a viscosifying agent with atleast a cyclodextrin modifier wherein the viscosity of the treatmentfluid is reduced.

In another embodiment, the present invention provides a method ofincreasing viscosity of an aqueous fluid comprising contacting at leasta hydrophobically modified polymer and a cyclodextrin modifier in thepresence of at least the aqueous fluid, wherein the viscosity of theaqueous fluid is increased.

In another embodiment, the present invention provides a subterraneantreatment fluid comprising a base fluid and a cyclodextrin modifier.

The features and advantages of the present invention will be readilyapparent to those skilled in the art. While numerous changes may be madeby those skilled in the art, such changes are within the spirit of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention, and should not be used to limit or define theinvention.

FIG. 1 illustrates the toroidal shape of γ-cyclodextrin.

FIG. 2 illustrates hydrophobic associations between HM polymers, inaccordance with embodiments of the present invention.

FIG. 3 illustrates use of a surfactant to promote hydrophobicassociations between HM polymers, in accordance with embodiments of thepresent invention.

FIG. 4 illustrates complexing of a cyclodextrin modifier with HMpolymers, in accordance with embodiments of the present invention.

FIG. 5 is a graph of viscosity versus concentration ofhydroxypropyl-β-cyclodextrin for sample fluids comprising a HM polymerand the hydroxypropyl-β-cyclodextrin.

FIG. 6 is a graph of apparent viscosity versus shear rate for samplefluids comprising a HM polymer, a surfactant, and/orhydroxypropyl-β-cyclodextrin.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to methods and compositions for use insubterranean operations. More particularly, in certain embodiments, thepresent invention relates to methods of altering the viscosity and/orfoaming characteristics of a subterranean treatment fluid with acyclodextrin modifier.

There may be several potential advantages to the methods andcompositions of the present invention. In accordance with embodiments ofthe present invention, a cyclodextrin modifier may be used to modify theviscosity of a treatment fluid. In one embodiment, a cyclodextrinmodifier may contact a treatment fluid comprising a viscosifying agent,which may include, inter alia, a viscoelastic surfactant, a combinationof a surfactant and a HM polymer, and/or a phosphonate surfactant, todecrease the viscosity of the treatment fluid. In other embodiments, acyclodextrin modifier may contact a treatment fluid comprising aviscosifying agent, which may include a HM polyampholyte, to increasethe viscosity of the treatment fluid.

I. Example Cyclodextrin Modifiers

One example of a suitable cyclodextrin modifier includes cyclodextrin.Generally, cyclodextrin is thought to be a cyclic oligosaccharidecomprising at least 6 glucopyranose units joined by α-(1,4) glycosidiclinkages. While cyclodextrins may have up to 150 or more glucopyranoseunits, the more common cyclodextrins comprise 6, 7, or 8 (α, β, and γ,respectively) glucopyranose units joined by α-(1,4) glycosidic linkages.As illustrated by FIG. 1, cyclodextrins comprising 6-8 glucopyranoseunits can be represented as toroids. In the illustrated embodiment,γ-cyclodextrin is represented as toroid 10 with larger opening 12 andsmaller opening 14 of toroid 10 representing secondary and primaryhydroxyl groups, respectively. In general, the exterior 16 of toroid 10should be sufficiently hydrophilic for the cyclodextrin to possess somewater solubility. Internal cavity 18 of toroid 10 is generally apolar orrelatively more hydrophobic and less hydrophilic than the exterior ofthe molecule and, thus, should be attractive to hydrophobic orlipophilic molecules. For example, the internal cavity (such as internalcavity 18) should be capable of hosting a hydrophobic portion of a“guest” compound, such as a surfactant or HM polymers, to form aninclusion complex therewith. As used herein, the term “inclusioncomplex” generally refers to the complex formed with the cyclodextrinfunctioning as a “host” to a “guest” compound that is contained orbound, wholly or partially, within the internal cavity of thecyclodextrin. It is believed that this entrapment of the hydrophobicportion of a surfactant or HM polymers should deactivate propertiesassociated with hydrophobic groups (portion), such as, for example,micelle formation, viscosification, shear thickening, etc. Accordingly,cyclodextrin may be used to modify the viscosity of a treatment fluid,for example.

Derivatives of cyclodextrins may also be suitable for use as thecyclodextrin modifiers in accordance with embodiments of the presentinvention. In general, cyclodextrin derivatives are also capable offorming inclusion complexes with a hydrophobic portion of a “guest”compound. A variety of different cyclodextrin derivatives may beprepared by introducing different functional groups into thecyclodextrin molecule by reaction with the primary hydroxyl groupsand/or the secondary hydroxyl groups. Because the hydroxyl groups havedifferent reactivity, derivatizing cyclodextrin may result in anamorphous mixture that includes numerous isomers of differentsubstituted cyclodextrin derivatives. Examples of suitable cyclodextrinderivatives include, but are not limited to: (1) acylated cylodextrincontaining acetyl, propionyl, butyryl, or other suitable acyl groups;(2) hydroxylated cyclodextrin containing hydroxyethyl, hydroxypropyl, orother suitable hydroxy-alkyl groups; (3) carboxylated cylcodextrincontaining carboxymethyl, carboxyethyl, or other suitable carboxyalkylgroups, and (4) alkylated cyclodextrin containing methyl, ethyl, propyl,benzyl, or other suitable alkyl groups. Examples of some of thesecyclodextrin derivatives include, but are not limited to, methylcyclodextrins, hydroxyethyl cyclodextrins, hydroxypropyl cyclodextrins,2-hydroxyethyl cyclodextrins, carboxymethyl cyclodextrins, andcarboxyethyl cyclodextrins. In certain embodiments, cyclodextrin mayhave glucose or maltose attached to the cyclodextrin ring, such asglucosyl cyclodextrins and maltosyl cyclodextrins. Specific examples ofsuitable cyclodextrin derivatives, include, but are not limited to,glucosyl-α-cyclodextrin, maltosyl-α-cyclodextrin,glucosyl-β-cyclodextrin, maltosyl-β-cyclodextrins,methyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin,hydroxyethlyl-β-cyclodextrin, and 2-hydroxypropyl-γ-cyclodextrin.Combinations of the above-described cyclodextrins may also be suitable.Suitable cyclodextrins are available from CTD, Inc., High Springs, Fla.

Oligomerized cyclodextrins (such as cyclodextrin dimers and cyclodextrintrimers) and polymerized cyclodextrins are also suitable for use ascyclodextrin modifiers in accordance with embodiments of the presentinvention. In general, these oligomerized and polymeric cyclodextrinmodifiers should also capable of forming inclusion complexes with ahydrophobic portion of a “guest” compound. Without being limited bytheory, it is believed that oligomerized and polymerized cyclodextrinsgenerally should be suitable for viscosification of HM polymers. Inaddition, it is believed that oligomerized and polymerized cyclodextrinsgenerally should also be suitable for decreasing the viscosity oftreatment fluids viscosified with a viscoelastic surfactant.Cyclodextrin dimers generally include two cyclodextrin moleculescovalently coupled or crosslinked together for cooperative complexingwith a “guest” compound. Cyclodextrin trimers generally include threecyclodextrin molecules covalently coupled or crosslinked together forcooperative complexing with a “guest” compound. Polymerizedcyclodextrins generally include a unit of 10 or more cyclodextrinmolecules covalently coupled or crosslinked together for cooperativecomplexing with a “guest” compound. Examples of suitable oligomerizedand/or polymerized cyclodextrins include, but are not limited to, thosecontaining carboxymethyl cyclodextrins, glucosyl cyclodextrins, maltosylcyclodextrins, hydroxypropyl cyclodextrins, and 2-hydroxypropylcyclodextrins.

II. Example Methods of Viscosity Reduction

As previously mentioned, a cyclodextrin modifier may be utilized toeffect a viscosity decrease in a treatment fluid, in accordance withembodiments of the present invention. By way of example, a treatmentfluid comprising a base fluid and a viscosifying agent may be contactedwith a cyclodextrin modifier, wherein the viscosity of the treatmentfluid is reduced. Without being limited by theory, it is believed thatthe cyclodextrin modifier interacts with the viscosifying agent toreduce the viscosity of the treatment fluid. It is further believed, forexample, that the cyclodextrin modifier hosts a hydrophobic portion ofthe viscosifying agent to form an inclusion complex therewith,deactivating properties of the viscosifying agent associated withhydrophobic associations, such as viscosification. By reducing theviscosity of the treatment fluid, the cyclodextrin modifier may functionsimilar to breakers commonly used, e.g., in fracturing, however, withoutbreaking a long-chain molecule into shorter segments.

In general, the treatment fluids that may have their viscosity decreasedby contact with a cyclodextrin modifier may comprise a base fluid and aviscosifying agent. The treatment fluid generally may include any of avariety of treatment fluids in which it may be desirable to facilitate areduction in viscosity, including HM polymer gels, viscoelasticsurfactant gels, and oil gels. Examples of suitable treatment fluidsinclude, but are not limited to, stimulation fluids (e.g., fracturingfluids, acidizing fluids, etc.), drilling fluids, completion fluids, andthe like. Additional additives may be included in the treatment fluidsas desired for a particular application, including, but not limited to,gel stabilizers, fluid-loss-control additives, clay stabilizers,bactericides, proppant particulates, gravel particulates, pH-adjustingagents, pH buffers, combinations thereof, and the like. For example,proppant particulates may be included in a fracturing fluid and maydeposited in fractures to prevent the fractures from closing so thatconductive channels may be formed through which produced hydrocarbonscan readily flow.

Suitable base fluids include aqueous fluids and oleaginous fluids.Examples of suitable aqueous fluids include, but are not limited to,freshwater, seawater, saltwater (water comprising a dissolved salt), andbrines. Where present, the oleaginous fluid may be from natural orsynthetic sources. Examples of suitable oleaginous fluids include, butare not limited to, α-olefins, internal olefins, alkanes, aromaticsolvents, cycloalkanes, liquefied petroleum gas, kerosene, diesel oils,crude oils, gas oils, fuel oils, paraffin oils, mineral oils,low-toxicity mineral oils, olefins, esters, amides, synthetic oils(e.g., polyolefins), polydiorganosiloxanes, siloxanes, organosiloxanes,ethers, acetals, dialkylcarbonates, hydrocarbons, and combinationsthereof.

Suitable viscosifying agents include viscosifying agents capable ofincreasing the viscosity of the treatment fluid through hydrophobicinteractions. The increased viscosity of the treatment fluid may, forexample, reduce fluid loss and allow transport of significant quantitiesof suspended particulates (e.g., gravel or proppant particulates).Examples of suitable viscosifying agents include, but are not limitedto, viscoelastic surfactants, HM polymers, phosphonate surfactants, andcombinations thereof. In general, the viscosifying agent may be includedin the treatment fluid in an amount sufficient to provide the desiredviscosity. For example, the viscosifying agent may be present in anamount of 0.01% to about 15% by weight of the treatment fluid and,alternatively, in an amount of 0.1% to about 10% by weight of thetreatment fluid.

A viscoelastic surfactant may increase the viscosity of the treatmentfluid, for example, by association of hydrophobic portions of thesurfactants to form micelles of specialized or larger associativestructures, for example worm-like structures. In certain embodiments,the viscoelastic surfactant is used in a treatment fluid with an aqueousbase fluid. Example of suitable viscoelastic surfactants, include, butare not limited to: anionic VES surfactants such as alkyl sarcosinate;cationic surfactants such as fatty amine salts orN-erucyl-M,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride;zwitterionic surfactants such as erucylamidopropyl betaine amine;non-ionic surfactants such as amidoamine oxides, amine oxides; andcombinations thereof. Because micelle formation may be dependent on anumber of factors, including pH of the treatment fluid, the treatmentfluid may contain additional additives, such as pH buffers and/orpH-adjusting agents. Where the viscosifying agent comprises aviscoelastic surfactant, the base fluid generally comprises an aqueousfluid. In certain embodiments, a viscoelastic surfactant may be used incombination with a HM polymer.

HM polymers also may be used to increase the viscosity of the treatmentfluid. As illustrated by FIG. 2, the HM polymers 20 may increase theviscosity, for example, by associative interactions between hydrophobicgroups 22 of the HM polymers 20 to form intermolecular micellar bonds,which result in a three-dimensional network 24. In addition, surfactantsmay be used to facilitate the formation of these micellar bonds. It isbelieved that the hydrophobic groups 22 of the HM polymers 24 may becomeincorporated into surfactant micelles 26, which act as crosslinkers, asillustrated by FIG. 3. These surfactants may show newtonian orviscoelastic behavior when present in water by themselves inconcentrations of less than 20%. In certain embodiments, the HM polymeris used in a treatment fluid with an aqueous base fluid.

While the HM polymers generally have hydrophobic groups incorporatedinto the hydrophilic polymer structure, the HM polymer should remainwater soluble. In certain embodiments, the HM polymer comprises ahydrophilic polymer backbone and alkyl branch of 4 to 22 carbons (e.g.,6 carbons, 8 carbons, 10 carbons, 12 carbons, 14 carbons, 16 carbons, 18carbons, 20 carbons). In certain embodiments, the HM polymer maycomprise a HM polyelectrolyte containing only one type of charge, forexample, a HM anionic or HM cationic polymer. Examples of suitable HMpolymers include, but are not limited to, HM polysaccharides andderivatives thereof that contain one or more of these monosaccharideunits: galactose; mannose; glucoside; glucose; xylose; arabinose;fructose; glucuronic acid; or pyranosyl sulfate. Examples of suitable HMpolysaccharides include, but are not limited to, HM guar gum andderivatives thereof, such as HM hydroxypropyl guar and HMcarboxymethylhydroxypropyl guar, and HM cellulose derivatives, such asHM hydroxyethyl cellulose and HM carboxymethyl cellulose. Additionally,synthetic HM polymers and HM copolymers that contain the above-mentionedfunctional groups may be used. Examples of such synthetic polymersinclude, but are not limited to, HM polyacrylate, HM polymethacrylate,HM polyacrylamide, HM polyvinyl alcohol, and HM polyvinylpyrrolidone.Combinations of HM polymers may also be suitable. An example of asuitable HM hydroxyethyl cellulose is available as Natrasol Plus® orNatrasol Plus® 330 from Hercules Inc., Wilmington, Del.

As previously mentioned, a surfactant may be used to facilitateviscosification of a fluid (such as the base fluid in the treatmentfluid) with a HM polymer. In certain embodiments, the surfactant may bea non-viscoelastic surfactant. Suitable surfactants capable offunctioning as crosslinkers for HM polymers may be anionic, neutral,cationic or zwitterionic. Aqueous liquids containing the surfactants mayrespond to shear with a newtonian or viscoelastic behavior. Anionicsurfactants with Newtonian rheological behavior are preferred. Examplesof suitable anionic surfactants include, but are not limited to, sodiumdecylsulfate, sodium lauryl sulfate, alpha olefin sulfonate, alkylethersulfates, alkyl phosphonates, alkane sulfonates, fatty acid salts,arylsulfonic acid salts, and combinations thereof. Examples of suitablecationic surfactants, include, but are not limited to,trimethylcocoammonium chloride, trimethyltallowammonium chloride,dimethyldicocoammonium chloride, bis(2-hydroxyethyl)tallow amine,bis(2-hydroxyethyl)erucylamine, bis(2-hydroxyethyl)coco-amine,cetylpyridinium chloride, and combinations thereof. Where used, thesurfactant may be included in the treatment fluid in an amount of about0.1% to about 20% by weight of the treatment fluid (e.g., 2%, 4%, 6%,8%, 10%, 12%, 14%, 16%, or 18%).

A phosphonate surfactant may be used to viscosify a treatment fluidcomprising an oleaginous base fluid, for example, by association of thehydrophobic portions of the phosphonate surfactant. In general, aphosphonate surfactant comprises a C—PO(OH)₂ or C—PO(OR)₂ group, whereinR is an alkyl or aryl group. Examples of suitable phosphonatesurfactants include, but are not limited to octylphosphonic acidmonomethyl ester monomethyl ester and combinations thereof. Examples ofsuitable phosphonate surfactants are described in U.S. Pat. No.6,511,944 and No. 6,544,934, the disclosure of which are incorporatedherein by reference.

As previously discussed, the suitable viscosifying agents generally mayincrease the viscosity of the treatment fluid through hydrophobicinteractions. For example, hydrophobic portions of the viscoelasticsurfactant may associate to form micelles or larger associativestructures, while hydrophobic portions of the HM polymer may associateto form what are considered micellar bonds, resulting in athree-dimensional network. In accordance with embodiments of the presentinvention, these treatment fluids may be contacted by a cyclodextrinmodifier. As previously mentioned, the cyclodextrin modifier includes aninternal cavity (such as internal cavity 18 on FIG. 1) that is capableof hosting a hydrophobic portion of a “guest” compound, such as theviscosifying agent, to form an inclusion complex therewith. Asillustrated by FIG. 4, the cyclodextrin modifiers 28 should host thehydrophobic groups 22 of the HM polymers 20, forming inclusion complexes30 therewith. It is believed that this entrapment of the hydrophobicportion of the viscosifying agent should deactivate propertiesassociated with hydrophobic associations that increase the viscosity ofthe fluid. Accordingly, the cyclodextrin modifier may be used todecrease the viscosity of the treatment fluid.

These viscosity-reduction methods may be initiated by contacting thetreatment fluid with at least the cyclodextrin modifier, in accordancewith embodiments of the present invention. Suitable modifiers for thepurpose of viscosity reduction are non-polymeric. To decrease theviscosity of the treatment fluid, the cyclodextrin modifier may be addedto the treatment fluid, for example, in an amount in the range of 0.01%to about 30% by weight of the treatment fluid (e.g., 0.05%, 0.1%, 0.5%,1%, 2%, 3%, 4%, 5%, 6%) and, alternatively, in the range of 1.0% toabout 20% by weight of the treatment fluid. In general, the maximumamount of the cyclodextrin modifier that may be used to decreaseviscosity may be equivalent to moles of hydrophobic groups present inthe viscosifying agent. However, a particular amount of the cyclodextrinmodifier may be the amount determined in the laboratory required toobtain maximum reduction in viscosity of the treatment fluid measured atthe temperature. In certain embodiments, the cyclodextrin modifier maycontact the treatment fluid subsequent to the introduction of thetreatment into a well bore. For example, the treatment fluid may beintroduced into a subterranean formation at or above a pressuresufficient to create or enhance one or more fractures in thesubterranean formation. After introduction of the treatment fluid intothe subterranean formation, it may be desirable to reduce the viscosityof the treatment fluid so that it may be recovered from the formationand/or proppant particulates may be deposited in the formation.Accordingly, a fluid comprising the cyclodextrin modifier may then beintroduced into the well bore such that the cyclodextrin modifiercontacts the treatment fluid. As previously mentioned, the cyclodextrinmodifier should then interact with the viscosifying agent to decreaseviscosity of the treatment. In certain embodiments, a spacer fluid maybe introduced into the well bore between the treatment fluid comprisingthe viscosifying agent and the fluid comprising the cyclodextrinmodifier. In certain embodiments, the cyclodextrin modifier may havebeen introduced into the well bore prior to the treatment fluid. Inthese embodiments, the treatment fluid should contact the cyclodextrinmodifier after its introduction into the well bore. For example, afterintroduction of the treatment fluid, the well bore may be placed ondrawdown to recover the treatment fluid. When the well bore is placed ondrawdown, the cyclodextrin modifier that has preceded the treatmentfluid should flow back to the well bore contacting the treatment fluidin the well bore.

III. Example Methods of Viscosification

In addition, cyclodextrin may be utilized to effect a viscosity increasein a treatment fluid, in accordance with embodiments of the presentinvention. For example, a treatment fluid comprising a base fluid and aHM polymer may be contacted with a cyclodextrin modifier, wherein theviscosity of the treatment fluid is increased. Without being limited bytheory, it is believed that the cyclodextrin modifier interacts with theHM polymer to viscosify the treatment fluid. In certain embodiments,viscosification may be accomplished using polymeric cyclodextrins and HMpolymers. In certain embodiments, viscosification may be accomplishedusing a cyclodextrin modifier and a HM polyampholyte. As used herein,the term “polyampholyte” refers to a polymer molecule containing bothanionic and cationic groups in the polymer chain. Without being limitedby theory, it is believed that the monomeric and oligomeric cyclodextrinmodifiers viscosify by deactivating intramolecular and intermolecularassociative bonding in the polymer by hosting the hydrophobic groups andthus allowing ionic associations among oppositely charged ionic groupspresent on different polymer chains. It is also believed, without beinglimited by theory, that deactivation of intramolecular hydrophobicassociative bonds allows for uncoiling of the polymer chains andincreasing the degree of intermolecular ionic bonding. Use of polymericcyclodextrin modifiers would provide for additional crossing owing tothe crosslinking of the HM polymer chains by cyclodextrin groups in thepolymeric cyclodextrin hosting the hydrophobic chains on the HM polymer.When polymeric cyclodextrin modifiers are used for viscosification, theHM polymers need not contain oppositely charged ionic groups and may benon-ionic or may contain either anionic, cationic or zwitterionicgroups.

In certain embodiments, the treatment fluids that may have theirviscosity increased by contact with a cyclodextrin modifier may comprisean aqueous fluid and a HM polyampholyte. Examples of suitable aqueousfluids include, but are not limited to, freshwater, seawater, saltwater(water comprising a dissolved salt), and brines. The treatment fluidgenerally may include any of a variety of treatment fluids in which itmay be desirable to facilitate an increase in viscosity. Examples ofsuitable treatment fluids include, but are not limited to, stimulationfluids (e.g., fracturing fluids, acidizing fluids, etc.), drillingfluids, completion fluids, conformance gels, relative-permeabilitymodifiers, gravel-pack fluids and the like. Additional additives may beincluded in the treatment fluids as desired for a particularapplication, including, but not limited to, gel stabilizers,fluid-loss-control additives, clay stabilizers, bactericides, proppantparticulates, gravel particulates, pH-adjusting agents, pH buffers,combinations thereof, and the like. For example, proppant particulatesmay be included in a fracturing fluid and may be deposited in fracturesto prevent the fractures from closing so that conductive channels may beformed through which produced hydrocarbons can readily flow. By way ofanother example, a drilling fluid carrying a drill cuttings andcirculated to the surface subsequent to drilling may be made to releasethe cuttings by reducing the viscosity of the fluid by the addition of acyclodextrin modifier to the fluid.

In general, the HM polyampholyte may be included in the treatment fluidin an amount sufficient for a particular application. For example, theHM polyampholyte may be present in an amount of 0.02% to about 10% byweight of the treatment fluid (e.g., 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%,5%, 6%, 7%, 8%, or 9%).

While the HM polyampholyte generally has hydrophobic groups incorporatedinto the hydrophilic polymer structure, the HM polyampholyte shouldremain water soluble. In certain embodiments, the HM polyampholytecomprises a hydrophilic polymer backbone and an alkyl branch of 4 to 22carbons (e.g., 6 carbons, 7 carbons, 8 carbons, 10 carbons, 12 carbons,14 carbons, 16 carbons, 18 carbons, 20 carbons). Examples of suitable HMpolyampholytes include, but are not limited to, HM cationic partiallyhydrolyzed polyacrylamides, HM poly(vinylamines/acrylic acid), HM alkylacrylate polymers with pendant amine groups in general, and combinationsthereof. The anionic groups in these HM polyampholyte may be due, forexample, to partial hydrolysis of the acrylate groups. Additionalexamples of HM alkyl acrylate polymers include, but are not limited to,HM polydimethylaminoethyl methacrylate, polydimethylaminopropylmethacrylamide, HM poly(acrylamide/dimethylaminoethyl methacrylate), HMpoly(methacrylic acid/dimethylaminoethyl methacrylate), HMpoly(2-acrylamido-2-methyl propane sulfonic acid/dimethylaminoethylmethacrylate), HM poly(acrylamide/dimethylaminopropyl methacrylamide),HM poly (acrylic acid/dimethylaminopropyl methacrylamide), HMpoly(methacrylic acid/dimethylaminopropyl methacrylamide), andcombinations thereof. In certain embodiments, the HM polyampholytecomprises a polymer that has been hydrophobically modified with an alkylgroup present on an amino group (in the polymer backbone or as a pendantgroup) in quaternized form. For example, an alkyl group may be presenton a dialkyl amino pendant group in quaternized form. In one embodiment,the dialkyl amino pendant group comprises a dimethyl amino pendantgroup. Specific examples of suitable HM polyampholytes include, but arenot limited to a polydimethylaminoethylmethacrylate orpolydimethylaminopropylmethacrylamide that has been hydrophobicallymodified with an alkyl group with 14 carbons to 22 carbons (e.g., 16carbons, 18 carbons, 20 carbons) on a dimethylamino group. An example ofa suitable HM polyampholyte is HPT-1™ relative permeability modifyingpolymer available from Halliburton Energy Services, Inc., Duncan, Okla.

As previously discussed, the HM polyampholyte present in the treatmentfluid may interact with the cyclodextrin modifier to increase theviscosity of the treatment fluid. In addition, the polyampholyte mayalso function as a relative permeability modifier. As used herein, theterm “relative permeability modifier” refers to a polymer thatselectively reduces the effective permeability of a subterraneanformation to water. It is believed that the HM polyampholyte may attachto surfaces within the subterranean formation, reducing the formation'seffective permeability to water without a comparable reduction in itspermeability to hydrocarbons

As previously mentioned, viscosification of treatment fluids usingcyclodextrin modifiers can also be accomplished by the use of polymericcyclodextrins and HM polymers. For example, a treatment fluid comprisinga base fluid and a HM polymer may be contacted with a polymericcyclodextrin, wherein the polymeric cyclodextrin interacts with the HMpolymer to viscosify the treatment fluid. In these embodiments, the HMpolymer need not be a HM polyampholyte. For example, the HM polymers maybe non-ionic, anionic, or cationic or may contain zwitterionic groups.Examples of suitable HM polymers suitable for use in combination withpolymeric cyclodextrin modifiers are described above in Section, but maybe used in certain embodiments without the surfactants.

These viscosification methods may be initiated by contacting thetreatment fluid with at least one cyclodextrin modifier, in accordancewith embodiments of the present invention. To increase the viscosity ofthe treatment fluid, the cyclodextrin modifier may be added to thetreatment fluid, for example, in an amount in the range of 0.01% toabout 10% by weight of the treatment fluid (e.g., 0.05%, 0.1%, 0.5%, 1%,2%, 3%, 4%, 5%, 6%) and, alternatively, in the range of 0.01% to about5% by weight of the treatment fluid. For example, a sufficientconcentration of the cyclodextrin modifier may be included in thetreatment to deactivate substantially all of the hydrophobic groups inthe polymer. In certain embodiments, the cyclodextrin modifier maycontact the treatment fluid prior to its introduction into a well bore.For example, the cyclodextrin modifier may be included in the treatmentfluid during its preparation and, thereafter, the treatment fluid may beintroduced into the well bore. For example, a treatment fluid comprisinga base fluid, a HM polymer, and a cyclodextrin modifier may beintroduced into the well bore. As previously mentioned, the HM polymerand the cyclodextrin modifier may interact to increase the viscosity ofthe treatment fluid. In certain embodiments, the treatment fluid may beintroduced into a subterranean formation at or above a pressuresufficient to create or enhance one or more fractures in thesubterranean formation.

In certain embodiments, the cyclodextrin modifier may contact a HMpolyampholyte subsequent to its introduction into the subterraneanformation. For example, the treatment fluid comprising the aqueous fluidand the HM polyampholyte may be introduced into a subterraneanformation. In certain embodiments, the treatment fluid may be bullheadedinto the formation, in that the treatment fluid is introduced into thesubterranean formation without isolation of the treated portion of theformation. Those of ordinary skill in the art, with the benefit of thisdisclosure, will appreciate that the HM polymer may be introduced into aportion of the subterranean formation to reduce its effectivepermeability to water. In certain embodiments, water production from thesubterranean formation may be monitored. To facilitate viscosification,the HM polymer present in the subterranean formation may be contactedwith a cyclodextrin modifier in the presence of an aqueous fluid,wherein the HM polymer and the cyclodextrin modifier interact toincrease the viscosity of the aqueous fluid inside the formation.Advantageously, sequential introduction of the treatment fluidcomprising the HM polymer and the cyclodextrin modifier may allowplacement of a plugging gel in the formation without the need forisolation of the treated portion of the formation. The production ofwater from this portion of the formation may be further reduced by thisin situ viscosification that results, for example, in formation of aplugging gel. In certain embodiments, the viscosified aqueous fluid maybe used for the diversion of aqueous fluids that are subsequentlyintroduced into the well bore in a variety of subterranean operations,such as in acid-stimulation operations, injection operations,scale-inhibition operations, and clay-stabilization operations.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, theentire scope of the invention.

EXAMPLE 1

The following series of tests were performed to determine the effect ofa cyclodextrin modifier on a fluid viscosified with a HM polymer and/ora surfactant. To prepare viscous fluids, samples were prepared by mixingHM hydroxyethyl cellulose with an aqueous fluid. The HM hydroxyethylcellulose utilized in this example was Natrasol Plus® hydroxyethylcellulose (“HEC”), available from Hercules, Inc. Further, the aqueousfluid used was tap water unless otherwise indicated. In certain samples,a surfactant (sodium dodecylsulfate) was also added to the aqueousfluid. After preparation, the viscosity of each sample was determinedusing a Brookfield viscometer at 12 rpm using a #2 or #3 spindle. Thedetermined viscosities for each sample are listed below in Table 1. Theconcentrations of the components shown in the table below are percent byweight of the aqueous fluid.

TABLE 1 Sample Viscosity (cP) 2% HM hydroxyethyl cellulose 3,800 1% HMhydroxyethyl cellulose 270 0.5% HM hydroxyethyl cellulose 20 1% HMhydroxyethyl cellulose + 725 0.0033% sodium dodecylsulfate 1% HMhydroxyethyl cellulose + 1,150 0.0283% sodium dodecylsulfate 1% HMhydroxyethyl cellulose + 23,500 0.283% sodium dodecylsulfate (lippinggel)

As illustrated by Table 1, an aqueous fluid may be viscosified using HMhydroxyethyl cellulose, either alone or in combination with asurfactant. As discussed previously, a cyclodextrin modifier may be usedto complex with a HM polymer, such as HM hydroxethyl cellulose,resulting in a fluid with reduced viscosity.

To demonstrate viscosity reduction with a cyclodextrin modifier offluids viscosified with a HM polymer and a surfactant, samples wereprepared by adding hydroxypropyl-β-cyclodextrin at differentconcentrations to a fluid containing 1% HM hydroxyethyl cellulose(Natrasol Plus® hydroxyethyl cellulose) and sodium dodecylsulfate. Inone sample, sodium dodecylsulfate and hydroxypropyl-β-cyclodextrin wereadded to a fluid containing 1% HM hydroxyethyl cellulose. Afterpreparation, the viscosity of each sample was determined using aBrookfield viscometer at 12 rpm using a #2 or #3 spindle. The determinedviscosities for each sample are listed below in Table 2. Theconcentrations of the components shown in the table below are percent byweight of the aqueous fluid.

TABLE 2 Sample Viscosity (cP) 1% HM hydroxyethyl cellulose + 1,4000.0283% sodium dodecylsulfate 1% HM hydroxyethyl cellulose + 700 0.0283%sodium dodecylsulfate + 0.095% hydroxypropyl-β-cyclodextrin 1% HMhydroxyethyl cellulose + 410 0.0283% sodium dodecylsulfate + 0.2%hydroxypropyl-β-cyclodextrin 1% HM hydroxyethyl cellulose + 620 mixtureof 0.0283% sodium dodecylsulfate and 0.2% hydroxypropyl-β-cyclodextrin

Accordingly, Table 2 demonstrates that the addition of a cyclodextrinmodifier, such as hydroxypropyl-β-cyclodextrin, to a viscosified fluidof a surfactant and a HM polymer results in a viscosity decrease.

To demonstrate viscosity reduction with a cyclodextrin modifier offluids viscosified with a HM polymer, samples were prepared by addinghydroxypropyl-β-cyclodextrin at different concentrations to a fluidcontaining 1% HM hydroxyethyl cellulose (Natrasol Plus® hydroxyethylcellulose). In certain sample, sodium dodecylsulfate was added to thismixture. After preparation, the viscosity of each sample was determinedusing a Brookfield viscometer at 12 rpm using a #2 or #3 spindle. Thedetermined viscosities for each sample are listed below in Table 3. Theconcentrations of the components shown in the table below are percent byweight of the aqueous fluid. A chart of viscosity versus %hydroxypropyl-β-cyclodextrin is provided in FIG. 5.

TABLE 3 Sample Viscosity (cP) 1% HM hydroxyethyl cellulose 270 1% HMhydroxyethyl cellulose + 200 0.0467% hydroxypropyl-β-cyclodextrin 1% HMhydroxyethyl cellulose + 100 0.0967% hydroxypropyl-β-cyclodextrin 1% HMhydroxyethyl cellulose + 50 0.133% hydroxypropyl-β-cyclodextrin 1% HMhydroxyethyl cellulose + 50 0.21% hydroxypropyl-β-cyclodextrin Mixtureof 1% HM hydroxyethyl cellulose and 600 0.133%hydroxypropyl-β-cyclodextrin + 0.033% sodium dodecylsulfate Mixture of1% HM hydroxyethyl cellulose and 2,050 0.133%hydroxypropyl-β-cyclodextrin + 0.083% sodium dodecylsulfate

Accordingly, Table 3 and FIG. 3 demonstrate that the addition of acyclodextrin modifier, such as hydroxypropyl-β-cyclodextrin, to a fluidviscosified with an HM polymer results in a viscosity decrease. Inaddition, Table 3 also demonstrates the fluid can be re-viscosified byaddition of sodium dodecylsulfate. This suggests that the complex formedbetween the cyclodextrin modifier and the HM polymer disassociates dueto stronger binding of the cyclodextrin modifier with the sodiumdodecylsulfate.

The rheological behavior of certain samples from this example was alsoevaluated. A chart of apparent viscosity versus shear rate for eachsample is provided in FIG. 6. FIG. 6 demonstrates the effectiveness ofhydroxypropyl-β-cyclodextrin in reducing viscosity in that the fluidwith a higher level of hydroxypropyl-β-cyclodextrin has almost identicalflow behavior as the fluid with only the HM polymer.

EXAMPLE 2

To demonstrate viscosity increase with a cyclodextrin modifier, a fluidwas prepared by mixing a cyclodextrin modifier with a HM polyampholyte.The first sample was prepared by mixing 30 mg solidhydroxypropyl-β-cyclodextrin with 5.3 grams of a 0.5% by weight solutionof a HM polyampholyte (HPT-1™ relative permeability modifying polymerfrom Halliburton Energy Services, Inc.). Upon shaking the test tube, thesample was observed to quickly form a non-flowable gel.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the present invention. In particular, every range of values(of the form, “from about a to about b,” or, equivalently, “fromapproximately a to b,” or, equivalently, “from approximately a-b”)disclosed herein is to be understood as referring to the power set (theset of all subsets) of the respective range of values, and set forthevery range encompassed within the broader range of values. Moreover,the indefinite articles “a” or “an”, as used in the claims, are definedherein to mean one or more than one of the element that it introduces.Also, the terms in the claims have their plain, ordinary meaning unlessotherwise explicitly and clearly defined by the patentee.

1.-28. (canceled)
 29. A method comprising: providing a cyclodextrinmodifier; providing a treatment fluid that comprises a base fluid and aviscosifying agent; wherein the viscosifying agent is selected from thegroup consisting of a hydrophobically modified polymer, a viscoelasticsurfactant, a phosphonate surfactant, or a combination thereof; and,introducing the cyclodextrin modifier and the treatment fluid into awell bore penetrating a subterranean formation wherein the viscosity ofthe treatment fluid is decreased due to the combination of thehydrophobically modified polymer and the cyclodextrin modifier.
 30. Themethod of claim 29 wherein the cyclodextrin modifier is not polymeric.31. The method of claim 29 wherein the cyclodextrin modifier is selectedfrom the group consisting of a methyl cyclodextrin, a hydroxyethylcyclodextrin, a hydroxypropyl cyclodextrin, a 2-hydroxyethylcyclodextrin, a carboxymethyl cyclodextrin, a carboxyethyl cyclodextrin,a glucosyl cyclodextrin, and a maltosyl cyclodextrin.
 32. The method ofclaim 29 wherein the cyclodextrin modifier is present in an amount ofabout 1% to about 20% by weight of the treatment fluid.
 33. The methodof claim 29 wherein the cyclodextrin modifier is selected from the groupconsisting of glucosyl-α-cyclodextrin, maltosyl-α-cyclodextrin,glucosyl-β-cyclodextrin, maltosyl-α-cyclodextrins,methyl-α-cyclodextrin, hydroxypropyl-β-cyclodextrin,2-hydroxypropyl-β-cyclodextrin, hydroxyethyl-α-cyclodextrin, and2-hydroxypropyl-γ-cyclodextrin.
 34. The method of claim 29 wherein theviscosifying agent is selected from the group consisting of ahydrophobically modified hydroxypropyl guar, a hydrophobically modifiedcarboxymethylhydroxypropyl guar, a hydrophobically modified cellulosederivative, a hydrophobically modified polymethacrylate, ahydrophobically modified polyacrylamide, a hydrophobically modifiedpolyvinyl alcohol, a hydrophobically modified polyvinylpyrrolidone, andcombinations thereof.
 35. The method of claim 29 wherein theviscosifying agent is present in an amount of about 0.1% to about 10% byweight of the treatment fluid.
 36. A method comprising: providing atreatment fluid that comprises a base fluid and a viscosifying agent;wherein the viscosifying agent is selected from the group consisting ofa hydrophobically modified polymer, a viscoelastic surfactant, aphosphonate surfactant, or a combination thereof; and, introducing thetreatment fluid into a well bore penetrating a subterranean formation ata pressure sufficient to create or enhance at least one fracture in thesubterranean formation; introducing a cyclodextrin modifier into theportion of the subterranean formation such that the cyclodextrinmodifier contacts the treatment fluid wherein the viscosity of thetreatment fluid is decreased due to the combination of thehydrophobically modified polymer and the cyclodextrin modifier.
 37. Themethod of claim 36 wherein the cyclodextrin modifier is not polymeric.38. The method of claim 36 wherein the cyclodextrin modifier is selectedfrom the group consisting of a methyl cyclodextrin, a hydroxyethylcyclodextrin, a hydroxypropyl cyclodextrin, a 2-hydroxyethylcyclodextrin, a carboxymethyl cyclodextrin, a carboxyethyl cyclodextrin,a glucosyl cyclodextrin, and a maltosyl cyclodextrin.
 39. The method ofclaim 36 wherein the cyclodextrin modifier is present in an amount ofabout 1% to about 20% by weight of the treatment fluid.
 40. The methodof claim 36 wherein the cyclodextrin modifier is selected from the groupconsisting of glucosyl-α-cyclodextrin, maltosyl-α-cyclodextrin,glucosyl-β-cyclodextrin, maltosyl-α-cyclodextrins,methyl-α-cyclodextrin, hydroxypropyl-β-cyclodextrin,2-hydroxypropyl-β-cyclodextrin, hydroxyethlyl-β-cyclodextrin, and2-hydroxypropyl-γ-cyclodextrin.
 41. The method of claim 36 wherein theviscosifying agent is selected from the group consisting of ahydrophobically modified hydroxypropyl guar, a hydrophobically modifiedcarboxymethylhydroxypropyl guar, a hydrophobically modified cellulosederivative, a hydrophobically modified polymethacrylate, ahydrophobically modified polyacrylamide, a hydrophobically modifiedpolyvinyl alcohol, a hydrophobically modified polyvinylpyrrolidone, andcombinations thereof.
 42. The method of claim 36 wherein theviscosifying agent is present in an amount of about 0.1% to about 10% byweight of the treatment fluid.
 43. The method of claim 36 wherein thecyclodextrin modifier is introduced to the subterranean formation beforethe treatment fluid is placed.
 44. The method of claim 36 wherein thecyclodextrin modifier is introduced to the subterranean formation afterthe treatment fluid is placed.